OK, I searched for 5 minutes, and found nothing. I lost/misplaced the wing nuton my 67 2 carb original air cleaner. I figured it would be a 1/4 - 20 thread and when going to a couple of stores to get a new one, the 1/4 - 20 doesn't fit.

What is the thread size for this wing nut? The threads on the stud don't look jacked up, and the one I lost fit just fine.

I lost/misplaced the wing nut. So I got a 1/4-20 nut to check the size and it goes on about one turn, then binds. That same nut fits on a known 1/4 - 20 bolt. I tried a 1/4-28 and it doesn't even start. Both the nuts were new and fit on a corresponding threaded bolt. The threads on the air cleaner stud don't really look jacked up. But what other size could it be. Just for grins, tried a m6 and m7. First too small, 2nd too big.

Before rethreading the stud I wanted to know if anyone can confirm that it's supposed to be a 1/4 - 20 thread size.

It sounds like your new wing nut is the "international standard"Try running a tap thru the wingnut and leave the original stud alone.Your original will probably turn up when you take the top shroud off!!